The scientific reason why Barack Obama and Mark Zuckerberg wear the same outfit every day

Barack Obama's days are packed with decisions that come from running the most powerful country on Earth.

Mark Zuckerberg's days are packed with decisions that come from running the most powerful social network on Earth.

They each wear basically the same thing every day: Obama a blue or gray suit, Zuckerberg a gray T-shirt.

And they do it for the same reason: decisions.

As in, they have to make tons of decisions all the time. And there's only so much mental energy in a given day, even if you're the leader of the free world or a wunderkind Harvard dropout.

Psychologists call it decision fatigue.

"Making decisions uses the very same willpower that you use to say no to doughnuts, drugs, or illicit sex," says Roy F. Baumeister, a psychologist who studies decision fatigue and a co-author of "Willpower: Rediscovering the Greatest Human Strength."

"It's the same willpower that you use to be polite or to wait your turn or to drag yourself out of bed or to hold off going to the bathroom," Baumeister told the New York Times. "Your ability to make the right investment or hiring decision may be reduced simply because you expended some of your willpower earlier when you held your tongue in response to someone's offensive remark or when you exerted yourself to get to the meeting on time."

As Obama told Vanity Fair in 2012, managing your life as a president requires that you cut away the mundane, frustrating decisions like deciding what to wear - which people around the world fret over.

"You'll see I wear only gray or blue suits," he said. "I'm trying to pare down decisions. I don't want to make decisions about what I'm eating or wearing. Because I have too many other decisions to make."

Zuckerberg said basically the same thing last November.

"I really want to clear my life to make it so that I have to make as few decisions as possible about anything except how to best serve this community," he said, meaning he doesn't want to spend mental energy on deciding what to wear or what to eat for breakfast.

"I'm in this really lucky position, where I get to wake up every day and help serve more than a billion people. And I feel like I'm not doing my job if I spend any of my energy on things that are silly or frivolous about my life," he said.

Zuckerberg said he drew the always-wear-the-same thing philosophy from Obama and a certain other tech executive, Steve Jobs. Jobs wore blue jeans and a black turtleneck nearly every single day. It provided him a signature style - and a daily convenience.

But wearing the same thing every day doesn't mean you have to look like a Silicon Valley type.

For three years, art director Matilda Kahl has worn the same stylish white silk top and black pants every day.